Vinylidene fluoride copolymers comprising recurring units derived from trifluoroethylene monomers have been used extensively in the electronics packaging market due to their ease of processing, chemical inertness and attractive ferroelectric, piezoelectric, pyroelectric and dielectric properties.
As is well known, the term piezoelectric means the ability of a material to exchange electrical for mechanical energy and vice versa and the electromechanical response is believed to be essentially associated with dimensional changes during deformation or pressure oscillation. The piezoelectric effect is reversible in that materials exhibiting the direct piezoelectric effect (the production of electricity when stress is applied) also exhibit the converse piezoelectric effect (the production of stress and/or strain when an electric field is applied).
Ferroelectricity is the property of a material whereby this latter exhibits a spontaneous electric polarization, the direction of which can be switched between equivalent states by the application of an external electric field.
Pyroelectricity is the ability of certain materials to generate an electrical potential upon heating or cooling. Actually, as a result of this change in temperature, positive and negative charges move to opposite ends through migration (i.e. the material becomes polarized) and hence an electrical potential is established.
It is generally understood that piezo-, pyro-, ferro-electricity in copolymers of vinylidene fluoride with trifluoroethylene is related to a particular crystalline habit, so called beta-phase, wherein hydrogen and fluorine atoms are arranged to give maximum dipole moment per unit cell.
Copolymers comprising recurring units derived from vinylidene fluoride and trifluoroethylene are typically provided as semicrystalline copolymers which can be shaped or formed into semicrystalline, essentially unoriented and unstretched, thermoplastic film or sheet or tubular-constructed product via well known processing methods such as extrusion, injection moulding, compression moulding and solvent casting.
For instance, WO 2009/147030 (SOLVAY SOLEXIS S.P.A.) Dec. 10, 2009 discloses polymers comprising recurring units derived from vinylidene fluoride and trifluoroethylene, said polymers comprising end groups of formula —CF2H and/or —CF2CH3 in an amount of at least 60 mmoles per Kg of vinylidene fluoride recurring units, which exhibit improved flexibility and deformability while maintaining outstanding piezoelectric, ferroelectric, pyroelectric and dielectric properties to be suitable for use in electrical and electronic devices.
Also, U.S. Pat. No. 5,087,679 (DAIKIN INDUSTRIES LTD.) Feb. 11, 1992 discloses use as dielectrics of copolymers comprising from 60% to 79% by moles of recurring units derived from vinylidene fluoride, from 18% to 22% by moles of recurring units derived from trifluoroethylene and from 3% to 22% by moles of recurring units derived from chlorotrifluoroethylene.
However, the vinylidene fluoride copolymers of the prior art do not satisfy critical electrical requirements to meet the required performance criteria for highly demanding high power electronics and microelectronics applications.
To ensure proper operation of a high power electronic circuit, in particular of a high power electronic circuit in a miniature form, proper isolation must be ensured between adjacent conductors by means of well performing insulating dielectric polymeric materials. High voltage arcing and leakage currents represent typical problems encountered in high voltage circuits, in particular when operating at high frequencies.
To counter these effects, dielectric polymeric materials which may be advantageously shaped into thin films and sheets need to be developed which exhibit high values for breakdown voltage or dielectric strength.
As is known, breakdown voltage or dielectric strength of an insulator defines the minimum voltage that causes a portion of the insulator to become electrically conductive. In solid insulating materials, this usually creates a weakened path within the material by creating permanent molecular or physical changes by the sudden electric current.
Additionally, the vinylidene fluoride copolymers of the prior art suffer from poor adhesive strength to substrates, in particular to metal substrates, to be advantageously used in electronic applications.
There is thus still a need in the art for vinylidene fluoride copolymer materials which fulfil such antagonist requirements and are endowed with improved values for breakdown voltage or dielectric strength, while also exhibiting good or enhanced adhesive strength properties to substrates, in particular to metal substrates, and maintaining outstanding thermal stability values and piezoelectric, ferroelectric, pyroelectric and dielectric properties.